Currently, marijuana is classified as a Schedule 1 drug under federal law, which means that it is considered as dangerous and addictive as drugs such as heroin, with no medical value. The DEA did not indicate where it stands on the issue, but it has previously refused to reschedule marijuana since the law was enacted in 1970.

The letter comes as a response to a 2015 request from eight Democratic senators, including Sen. Elizabeth Warren (D-Mass.), who implored the DEA to reschedule marijuana so that researchers can study its medical benefits. Researchers are largely restricted from accessing Schedule 1 drugs; only eight research institutions were allowed to study the plant in 2015, another DEA document revealed.

"That number is totally insufficient to meet public health needs and to answer the number of [research] questions that pop up yearly," the Brookings Institution's John Hudak told The Washington Post.

Twenty-three states and the District of Columbia have legalized marijuana, either for medical or recreational purposes. And despite federal restrictions in the U.S., a growing body of international research has found that marijuana effectively treats a range of physical and mental health conditions, including pain, anxiety, PTSD, and epilepsy. The Food and Drug Administration apparently made a recommendation to the DEA regarding the rescheduling of marijuana, but the agency did not reveal details about the FDA's position.

The 25-page letter from the DEA is signed by the agency's acting chief, Chuck Rosenberg, who came under fire last year when he called medical marijuana "a joke." More than 150,000 people signed a petition for Rosenberg's resignation following his statement, and Sen. Kirsten Gillibrand (D-New York) told ATTN: that she "doesn't feel [Rosenberg has] done his homework" on the subject.

Between mounting evidence of marijuana's medical value and increasing public support for legalization (61 percent of Americans support legalization, a 2016 AP-NORC poll found), there appears to be a growing call for marijuana reform. While rescheduling wouldn't legalize marijuana, it would represent a major victory for reform advocates and could affect state-level legislation as 14 states will consider marijuana-related ballot initiates in November.